Optical components such as waveguides are generally designed to confine and direct the propagation of light waves for many applications. In applications that rely on the reflection and transmission of light, significant gains in performance can be made when highly reflective materials are used in combination with optically transmissive materials. For example, a step-index fiber optic is composed of a thin strand of concentric layers of optically transmissive materials, a central optical medium (i.e., the core) and a surrounding optical medium (i.e., the cladding), the latter having a lower index of refraction. Light is channeled through the core. During transmission, the light often travels to the boundary of the core and cladding, where it is reflected back towards the core by total internal reflection However, total internal reflection is not total, as some of the light is lost, for example, due to scatter induced by imperfections within the core or at the core/cladding boundary.
To reduce this loss, a reflective layer can be applied over the surface of the cladding along the length of the fiber optic. The reflective layer significantly increases the amount of light directed back to the core, and improves the overall light transmission through the fiber optic.
Ideally, the reflective layer used in optical components should possess a high reflectance characteristic over a broad spectrum of light and over all incidence angles of reflectance. Silver is one metal known to possess a high reflectance value. Silver has a reflectance of about 98% over the entire visible light spectrum at normal incidence. Silver also sustains a high reflectance of about 96% for off-normal light at near grazing incidence angles. In comparison, aluminum, a more commonly used reflective-layer material, possesses a reflectance of about 93% at normal incidence. The reflectance of aluminum drops precipitously to 75% for light at grazing incidence angles
Although silver possesses excellent optical characteristics, there are several problems associated with the use of the reflective metal. Silver has a tendency to undesirably tarnish when exposed to the atmosphere, especially in the presence of corrosive gases and contaminants, including sulfur dioxide, hydrogen sulfide, nitrogen dioxide, ozone, hydrogen chloride, chlorine, and organic acids. It is known that long-term performance of silver coatings is rarely, if ever, guaranteed by commercial coating facilities based on the aggressive nature of silver tarnishing brought on by ordinary exposure to the environment, along with the lack of suitably available protective measures which have been successfully tested under corrosive conditions.
Further, silver's adherence to optically transmissive substrate materials, including glass or polymeric materials such as polymethyl methacrylate, is moderate at best. Polymethyl methacrylate is a low-cost acrylic resin frequently used in the fabrication of optical components
The use of silver coatings alone or in multilayer configurations with other metals having magnetic properties is also known in the art for providing shielding from electromagnetic interference (EMI). However, the use of silver coatings for EMI shielding applications has been limited by the susceptibility of silver coating to tarnishing and delaminating from an underlying substrate.
For the foregoing reasons, there is a need for an optical construction having a highly reflective coating that adheres favorably to a range of optically transmissive materials and that possesses improved resistance against corrosion and tarnishing to provide improved optically effective performance and longer lasting operating life Further, there is a long felt need in the art for methods and apparatus to provide highly conductive coatings or layers, typically comprising silver, possessing reduced susceptibility to tarnishing and/or delamination from an underlying supporting substrate, thereby enabling such highly conductive coatings or layers to be reliably used and maintained in EMI shielding applications.